As a conventional microwave heating apparatus of this type, for example, there is a high-frequency heating apparatus disclosed in Japanese Unexamined Patent Publication No. 56-132793. This conventional high-frequency heating apparatus includes a seed oscillator formed with using a semiconductor element, a divider for dividing an output of this seed oscillator into a plurality of outputs, a plurality of amplifiers for respectively amplifying the divided outputs, a phase shifter provided in one of paths between the divider and the amplifiers, a synthesizer for synthesizing outputs of the amplifiers again, and a plurality of antennas. The synthesizer in this conventional high-frequency heating apparatus has the substantially same configuration as that of the divider and is used so that an input and an output are opposite to each other. This synthesizer is formed with using a 90-degree hybrid coupler formed by a microstrip line, a 180-degree hybrid coupler, or the like, and has two synthesizer outputs. Therefore, by controlling the phase shifter provided in one of the paths between the divider and the amplifiers, a power ratio between the two synthesizer outputs can be changed, or phases between the two synthesizer outputs can be changed to the same phases or the opposite phases.
As another conventional microwave heating apparatus, for example, there is a high-frequency heating apparatus disclosed in Japanese Unexamined Patent Publication No. 52-84543. In this conventional high-frequency heating apparatus, solid high-frequency oscillators operated independently from each other are provided on one or more of E surfaces serving as electric field surfaces in wall surfaces forming a waveguide, H surfaces serving as magnetic field surfaces, and short surfaces serving as end surfaces of the waveguide and being orthogonal to the E surfaces and the H surfaces. In this conventional high-frequency heating apparatus, by emitting microwaves into the waveguide via couplers of the solid high-frequency oscillators and supplying the microwaves to a heating chamber via the waveguide, impedance matching is easily performed.    PLT 1: Japanese Unexamined Patent Publication No. 56-132793    PLT 2: Japanese Unexamined Patent Publication No. 52-84543